A variety of coatings are applied to substrates with melt coating processes, i.e., processes where the material is heated to soften it such that it will flow and applying the material to a substrate where it is cooled to form a melt coated layer thereon.
A variety of methods for melt coating materials have been suggested. Illustrative examples include forward and reverse roll coating, slot die coating, extrusion draw (drop) die coating, extrusion contact die coating and extrusion calendar coating processes. Illustrative roll coating, slot die coating and extrusion calendar coating processes are described in “Handbook of Pressure Sensitive Adhesive Technology (3rd Edition)” edited by Donatas Satas (Satas & Associates, Warwick, R.I., 1999, pp. 896-937.
Low viscosity (i.e., less than about 50 Pa-s), low molecular weight (i.e., less than about 200,000 g/mol) hot melt coatable polymers such as waxes; hot melt glues/adhesives (e.g., polyamide, polyolefin and polyester); hot melt styrene block copolymer PSAs and low molecular weight drum pumpable (meth)acrylic ester PSAs are typically coated using roll coating and slot die coating methods. These materials are liquid-like in the molten state and do not require an extrusion pumping process to deliver the materials to the coating process. They may require an extrusion process to melt and/or mix the materials. The low coating weight limit for these methods is about 20 to about 25 g/m2 and is dependent on many factors (e.g., molecular weight, extensional/elongational viscosity and shear viscosity).
High viscosity (i.e., more than about 50 Pa-s), high molecular weight (i.e., more than about 200,000 g/mol) hot melt coatable polymers are not liquid-like in the melt state due to the presence of polymer chain entanglements and require extrusion processes for mixing, pumping and coating operations. One illustrative coating method for such materials includes extrusion of the molten polymer out of a slot die into a nip formed by 2 rolls (which may have a conformable, e.g., rubber coating) and a moving substrate. If the molten polymer is tacky, the melt can be dropped onto a roller having a release coated surface and then laminated via nipping of the polymer melt to the moving substrate with a second rubber-covered roll. A second extrusion coating method is calendar coating which is described in the Satas reference. This method is suited for relatively high thickness hot melt coatings, i.e., greater than about 0.004 inches (100 μm), is relatively slow and requires extremely high nip pressures (more than about 3000 psi (20.7 megaPascals)). In a calendar coating operation, the coating composition and substrate are passed through affixed gap or nip in which they are squeezed together. An illustrative example of such processing is disclosed in U.S. Pat. No. 3,783,072 (Korpman).
U.S. Pat. No. 4,167,914 (Mladota) discloses an extrusion coater, sometimes referred to as press coated, for hot melt adhesive formulations that employs an elongated slot coating head with a small diameter rotating rod in cooperation with a larger diameter rotating compressible press roll. The rod and press roll are each rotated at predetermined speeds by motor. FIG. 1 is a derivation of a portion of FIG. 2 of U.S. Pat. No. 4,167,914 and shows some of the components shown there including elongated coating head 29 with chamber 45 connected to conduit 36 so that a supply of the viscous, hot melt, coating is pumped thereinto by a motorized pump (not shown) at a predetermined pressure and rate to meter the exact amount for extrusion through the elongated, narrow, extrusion slot 28 formed by lead-on lip, or edge, 46 and lead-off lip, or edge, 47. Web 22 is trained under the rubber like resilient surface 48 of press roll 49 which is supported, e.g., by pistons of air cylinders, to exert a predetermined pressure while also being yieldable. Cooperable with resilient surface 48 of press roll 49 and forming press roll nip 53 therewith is an elongated cylindrical rod 54 preferably having a hard smooth surface, the rod being rotatably seated for its major volume in groove 57 of substantially cylindrical cross-section in adjacent surface 58 of lead-off or downstream lip 47. Rod 54 is deeply seated in groove 57 such that only about the upper quarter of its volume is exposed. Rod 54 is mounted in its groove so that the pump pressure and rate causes a layer of coating composition to be applied to the face of web 22 as it is advanced under predetermined longitudinal and lateral tension from lead-on or upstream edge 46 to the curved hard face of rotating rod 54, the rod performing no metering function because it is revolved at relatively slow speed such as 9 to 12 rpm while the press roll and web advance at a considerably higher speed.
Other examples of web coating methods employing rotary rods are disclosed in U.S. Pat. No. 4,465,015 (Osta et al.) wherein a bar spaced from and trailing the delivery slit of the extrusion head is used as a spreader; U.S. Pat. No. 4,765,272 (Mladota) wherein a rotary doctor member, apparently not driven, is used to facilitate spreading of low viscosity compositions; and U.S. Pat. No. 4,757,782 (Pullinen) wherein a counter driven, i.e., in tangentially opposite direction, rotary rod is used as a smoothing device for low viscosity compositions.
Adhesives normally bond by surface attachment to other substances including various adherends and other adhesives. Pressure sensitive adhesives (PSAs) exhibit long term, preferably permanent, tackiness at ambient temperatures and will adhere to a variety of substrates upon application of light pressure, e.g., by finger or hand roller. An accepted quantitative description of a PSA is given by the Dahlquist criterion (see Handbook of Pressure-Sensitive Adhesive Technology, Second Edition, D. Satas, ed., Van Nostrand Reinhold, New York, N.Y., 1989, pages 171-176), which indicates that materials having a storage modulus (G′) of less than about 3×105 Pascals (measured at 10 rad/sec at room temperature, i.e., about 22° C.) have PSA properties, while materials having a G′ above this value do not. More specifically, a PSA as used herein refers to a material that has a storage modulus below the Dahlquist criterion at the use temperature, which may be different than room temperature.
PSAs are widely used in a vast array of applications. In a common configuration, coatings of PSA are provided on articles in desired locations. Illustrative examples include tabs, e.g., in sheet or strip form, or wound into roll form; self-adhesive sheets, e.g., note sheets, labels, sign sheeting, etc.; container closures; etc. A variety of adhesive formulations, offering a variety of performance, cost, and production criteria, have come to be used. Illustrative examples of materials from which PSA are made include acrylic polymers, thermoplastic elastomers, natural rubbers and synthetic rubbers, nonelastomeric thermoplastic polymers, etc.
Illustrative examples of methods for making PSA coatings or forming them on a desired surface include extrusion and solvent coating. Selection of a suitable method will be dependent in part upon the nature of the PSA formulation being used, nature of the substrate on which the coating is to be formed, available equipment and materials, etc.
Though various methods of making hot melt coatings are known, they are capable of producing satisfactory coating quality, i.e., as continuous as desired, only of coatings above certain thicknesses. The need exists to produce, at higher speeds, high quality melt coatings having lower thickness.